Method for manufacturing oxide semiconductor film and method for manufacturing semiconductor device

ABSTRACT

An object is to provide an oxide semiconductor having stable electric characteristics and a semiconductor device including the oxide semiconductor. A manufacturing method of a semiconductor film by a sputtering method includes the steps of holding a substrate in a treatment chamber which is kept in a reduced-pressure state; heating the substrate at lower than 400° C.; introducing a sputtering gas from which hydrogen and moisture are removed in the state where remaining moisture in the treatment chamber is removed; and forming an oxide semiconductor film over the substrate with use of a metal oxide which is provided in the treatment chamber as a target. When the oxide semiconductor film is formed, remaining moisture in a reaction atmosphere is removed; thus, the concentration of hydrogen and the concentration of hydride in the oxide semiconductor film can be reduced. Thus, the oxide semiconductor film can be stabilized.

TECHNICAL FIELD

One embodiment of the present invention relates to methods formanufacturing an oxide semiconductor film and a semiconductor deviceusing an oxide semiconductor.

BACKGROUND ART

A thin film transistor (a TFT) including a semiconductor thin film(having a thickness of several nanometers to several hundredsnanometers) which is formed over a substrate having an insulatingsurface is applied to a thin film integrated circuit, a liquid crystaldisplay device, or the like. In particular, application of a thin filmtransistor as a switching element provided in a pixel of a liquidcrystal display device is expanding.

In a conventional technique, a thin film transistor is manufacturedusing a silicon semiconductor. However, in recent years, a technique formanufacturing a thin film transistor using a metal oxide havingsemiconductor characteristics has attracted attention. Indium oxide is awell-known material as a metal oxide, and has been used as a transparentelectrode material which is necessary for a liquid crystal displaydevice because it has high conductivity.

On the other hand, it is known that a metal oxide shows semiconductorcharacteristics by control of the composition of the metal oxide;typically, tungsten oxide, tin oxide, indium oxide, zinc oxide, or thelike can be given as a metal oxide having semiconductor characteristics.Thin film transistors in which a channel formation region is formedusing such a metal oxide having semiconductor characteristics (i.e., anoxide semiconductor) are already known (Patent Documents 1 to 4,Non-Patent Document 1).

As metal oxides, multi-component oxides as well as single-componentoxides are known. For example, InGaO₃(ZnO)_(m) (m is a natural number)having a homologous series is known as a multi-component oxidesemiconductor including In, Ga, and Zn (Non-Patent Documents 2 to 4). Inaddition, it has been confirmed that an oxide semiconductor includingsuch an In—Ga—Zn-based oxide can be used as a channel layer of a thinfilm transistor (Patent Document 5, Non-Patent Documents 5 and 6).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. S60-198861

[Patent Document 2] Japanese Published Patent Application No. H8-264794

[Patent Document 3] Japanese Translation of PCT InternationalApplication No. H11-505377

[Patent Document 4] Japanese Published Patent Application No.2000-150900

[Patent Document 5] Japanese Published Patent Application No.2004-103957

Non-Patent Document

-   [Non-Patent Document 1] M. W. Prins, K. O. Grosse-Holz, G.    Muller, J. F. M. Cillessen, J. B. Giesbers, R. P. Weening, and R. M.    Wolf, “A ferroelectric transparent thin-film transistor”, Appl.    Phys. Lett., 17 Jun. 1996, Vol. 68, pp. 3650-3652-   [Non-Patent Document 2] M. Nakamura, N. Kimizuka, and T. Mohri, “The    Phase Relations in the In₂O₃—Ga₂ZnO₄—ZnO System at 1350° C.”, J.    Solid State Chem., 1991, Vol. 93, pp. 298-315-   [Non-Patent Document 3] N. Kimizuka, M. Isobe, and M. Nakamura,    “Syntheses and Single-Crystal Data of Homologous Compounds,    In₂O₃(ZnO)_(m) (m=3, 4, and 5), InGaO₃(ZnO)₃, and Ga₂O₃(ZnO)_(m)    (m=7, 8, 9, and 16) in the In₂O₃—ZnGa₂O₄—ZnO System”, J. Solid State    Chem., 1995, Vol. 116, pp. 170-178-   [Non-Patent Document 4] M. Nakamura, N. Kimizuka, T. Mohri, and M.    Isobe, “Syntheses and crystal structures of new homologous    compounds, indium iron zinc oxides (InFeO₃(ZnO)_(m)) (m: natural    number) and related compounds”, KOTAI BUTSURI (SOLID STATE PHYSICS),    1993, Vol. 28, No. 5, pp. 317-327-   [Non-Patent Document 5] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M.    Hirano, and H. Hosono, “Thin-film transistor fabricated in    single-crystalline transparent oxide semiconductor”, SCIENCE, 2003,    Vol. 300, pp. 1269-1272-   [Non-Patent Document 6] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M.    Hirano, and H. Hosono, “Room-temperature fabrication of transparent    flexible thin-film transistors using amorphous oxide    semiconductors”, NATURE, 2004, Vol. 432, pp. 488-492

DISCLOSURE OF INVENTION

However, because the composition of an oxide semiconductor cannot beeasily controlled, a difference from a stoichiometric composition of theoxide semiconductor occurs in a process of forming an oxidesemiconductor film. For example, electrical conductivity of an oxidesemiconductor is changed due to excess and deficiency of oxygen.Incorporation of hydrogen and moisture in a process of forming the oxidesemiconductor film form an O—H (oxygen-hydrogen) bond and serve as anelectron donor in the oxide semiconductor film, which results in changein electrical conductivity. Since an OH group is a polar molecule, itcauses change in characteristics of a thin film transistor manufacturedusing an oxide semiconductor film which includes a large number of OHgroups.

In view of the above problems, an object of one embodiment of thepresent invention is to provide an oxide semiconductor film havingstable electric characteristics and a semiconductor device using theoxide semiconductor film.

One embodiment of the present invention is a method for manufacturing asemiconductor film by a sputtering method, including the steps ofholding a substrate in a treatment chamber which is kept in areduced-pressure state; setting the temperature of the substrate tolower than 400° C.; introducing a sputtering gas from which hydrogen andmoisture are removed into the treatment chamber in the state whereremaining moisture in the treatment chamber is removed; and forming anoxide semiconductor film over the substrate with use of a metal oxide asa target.

When the oxide semiconductor film is formed, an entrapment vacuum pumpis preferably used for evacuating the treatment chamber. For example, acryopump, an ion pump, or a titanium sublimation pump is preferablyused. The vacuum pump such as a turbo molecular pump which evacuates gasby rotating a turbine blade at high speed has characteristics in thatthe pumping speed with respect to a light gas such as hydrogen isdecreased. Therefore, it is not suitable to use a turbo molecular pumpfor sufficiently evacuating remaining hydrogen and moisture from thetreatment chamber of a sputtering apparatus for manufacture of a highlypurified oxide semiconductor film. On the other hand, since anentrapment vacuum pump sufficiently evacuates even a light gas such ashydrogen, it is possible to reduce the concentration of hydrogen in theoxide semiconductor film to 5×10¹⁹/cm³ or less by removal of remaininghydrogen and moisture in the treatment chamber of the sputteringapparatus.

When the oxide semiconductor film is formed, a metal oxide containingzinc oxide as its main component can be used as the target.Alternatively, a metal oxide containing indium, gallium, and zinc can beused as the target.

One embodiment of the present invention is a method for manufacturing anoxide semiconductor film and a method for manufacturing a semiconductordevice which includes the step of forming a protective film forstabilizing characteristics of an element manufactured using the oxidesemiconductor film. In one embodiment of this manufacturing method, asubstrate over which a gate electrode and a gate insulating filmcovering the gate electrode are formed is introduced into a firsttreatment chamber; the temperature of the substrate is set to lower than400° C.; a sputtering gas from which hydrogen and moisture are removedis introduced in the state where remaining moisture in the firsttreatment chamber is removed; and an oxide semiconductor film is formedover the substrate with use of a metal oxide which is provided in thefirst treatment chamber and contains at least zinc oxide as a target.

Then, a source electrode and a drain electrode are formed over the oxidesemiconductor film; the substrate is introduced into a second treatmentchamber; the temperature of the substrate is set to lower than 100° C.;a sputtering gas including oxygen from which hydrogen and moisture areremoved is introduced in the state where remaining moisture in thesecond treatment chamber is removed; and a silicon oxide film includinga defect is formed over the substrate with use of a siliconsemiconductor target which is provided in the second treatment chamber.

Then, the substrate over which the silicon oxide film is formed isintroduced into a third treatment chamber; the substrate is heated at200° C. to 400° C. to diffuse hydrogen or moisture included in the oxidesemiconductor film into the silicon oxide film including the defect; anda silicon nitride film is formed over the silicon oxide film.

In the method for manufacturing a semiconductor device, when the oxidesemiconductor film and/or the silicon oxide film are/is formed, anentrapment vacuum pump is preferably used for evacuating the firsttreatment chamber and/or the second treatment chamber. For example, acryopump, an ion pump, or a titanium sublimation pump is preferablyused. The above entrapment vacuum pump functions so as to reducehydrogen, a hydroxyl group, or hydride included in the oxidesemiconductor film and/or the silicon oxide film.

In the method for manufacturing a semiconductor device, a metal oxidecontaining zinc oxide as its main component can be used as the targetfor forming the oxide semiconductor film. Alternatively, a metal oxidecontaining indium, gallium, and zinc can be used as the target.

In the method for manufacturing a semiconductor device, as the targetfor forming the silicon oxide film, a silicon semiconductor target or asynthetic quartz target can be used.

Note that the ordinal number such as “first”, “second”, or “third” usedto disclose or specify the present invention is given for convenience todistinguish elements, and not given to limit the number, thearrangement, and the order of the steps as long as there is noparticular limitation. When a component is mentioned as being “over” or“under” another component in order to disclose or specify the presentinvention, the two components are in direct contact with each other insome cases; however, another component may be present between the twocomponents in other cases. In terms used to disclose or specify thepresent invention, a singular can also mean a plural unless the meaningof the singular is clearly different from the meaning of the plural inthe context. The word “include” or “have” is used to express thepresence of a characteristic, a number, a step, operation, a component,a piece, or combination thereof, and do not eliminate the possibility ofpresence or addition of one or more other characteristics, numbers,steps, operation, components, pieces, or combination thereof. In theterms used to disclose or specify the present invention, all the termswhich are used including the technical or scientific terms have the samemeaning as ones which can be generally understood by those who haveconventional knowledge in the technical field to which the presentinvention belongs. The terms same as ones defined in a commonly-useddictionary should be interpreted as including the meaning in accordancewith the meaning in the context of the related art, and should not beinterpreted as being ideally or excessively literally unless they aredefined clearly in this specification.

When the oxide semiconductor film is formed, remaining moisture in areaction atmosphere is removed; thus, the concentration of hydrogen andthe concentration of hydride in the oxide semiconductor film can bereduced. Thus, the oxide semiconductor film can be stabilized.

When the oxide semiconductor film serving as a channel formation regionis formed over the gate insulating film, remaining moisture in areaction atmosphere is removed; thus, the concentration of hydrogen andthe concentration of hydride in the oxide semiconductor film can bereduced. By provision of the silicon oxide film including a defect incontact with the oxide semiconductor film, hydrogen and moisture in theoxide semiconductor film can be diffused into the silicon oxide film, sothat the concentration of hydrogen and the concentration of hydride inthe oxide semiconductor film can be reduced.

In addition, the silicon nitride film is formed over the silicon oxidefilm including a defect in the state where the substrate is heated;thus, hydrogen and moisture can be diffused from the oxide semiconductorfilm into the silicon oxide film and a barrier film for prevention ofentry of moisture from an outer atmosphere can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating a structure of asemiconductor device according to one embodiment.

FIGS. 2A to 2D are cross-sectional views illustrating a method formanufacturing a semiconductor device according to one embodiment.

FIG. 3 illustrates one example of a deposition apparatus.

FIG. 4 illustrates one example of a deposition apparatus.

FIG. 5 is a graph showing results of measuring the concentration ofhydrogen in oxide semiconductor films manufactured during evacuationwith a cryopump by a secondary ion mass spectrometry method.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the disclosed invention will be described. Note thatthe disclosed invention is not limited to the description below, and itis easily understood by those skilled in the art that a variety ofchanges and modifications can be made without departing from the spiritand scope of the disclosed invention. Therefore, the disclosed inventionshould not be interpreted as being limited to the following descriptionof the embodiment.

In the embodiment described below, the same reference numerals may beused to denote the same components among different drawings. Note thatelements in the drawings, that is, the thickness and width of layers,regions, the relative positional relationships between the components,and the like may be exaggerated for the sake of clarity of thedescription in the embodiment.

FIG. 1 illustrates a structure of a semiconductor device. In a thin filmtransistor 110 of the semiconductor device, a gate electrode 101 a and afirst wiring 101 b formed from the same layer as the gate electrode 101a are provided over a substrate 100. A gate insulating layer 102 isformed over the gate electrode 101 a and the first wiring 101 b. Thegate insulating layer 102 is preferably formed using an oxide insulatingmaterial.

FIG. 1 illustrates the case where the gate insulating layer 102 isformed of a first gate insulating layer 102 a and a second gateinsulating layer 102 b. In this case, the second gate insulating layer102 b in contact with an oxide semiconductor layer 123 is preferablyformed using an oxide insulating material. The oxide semiconductor layer123 is formed over the gate electrode 101 a with the gate insulatinglayer 102 therebetween.

A source electrode 104 a and a drain electrode 104 b are formed so thatend portions of the source electrode 104 a and the drain electrode 104 boverlap with the gate electrode 101 a. An oxide insulating film 105 isprovided over the source electrode 104 a and the drain electrode 104 b.Between the source electrode 104 a and the drain electrode 104 b, theoxide insulating film 105 is in contact the oxide semiconductor layer123. A protective insulating film 106 is provided over the oxideinsulating film 105.

A contact hole 108 is formed in the gate insulating layer 102 to reachthe first wiring 101 b. The first wiring 101 b and a second wiring 104 care connected to each other through the contact hole 108.

One example of a method for manufacturing a semiconductor device whichincludes the thin film transistor 110 will be described with referenceto FIGS. 2A to 2D.

FIG. 2A shows the stage at which the gate electrode 101 a, the firstwiring 101 b, the gate insulating layer 102, and an oxide semiconductorfilm 103 are formed over the substrate 100.

As a glass substrate used for the substrate 100, it is possible to use aglass substrate used for a liquid crystal panel. For example, a glassmaterial such as aluminosilicate glass, aluminoborosilicate glass, orbarium borosilicate glass can be used. More practical glass with heatresistance can be obtained by containing a larger amount of barium oxide(BaO) than the amount of boron oxide (B₂O₃). Therefore, a glasssubstrate containing a larger amount of BaO than the amount of B₂O₃ ispreferably used.

After formation of a conductive film over the substrate 100 having aninsulating surface, a first wiring layer including the gate electrode101 a and the first wiring 101 b is formed through a firstphotolithography step. End portions of the gate electrode 101 a arepreferably tapered. A resist mask may be formed by an inkjet method.Formation of the resist mask by an inkjet method needs no photomask;thus, manufacturing cost can be reduced.

As the conductive film for forming the gate electrode 101 a and thefirst wiring 101 b, an element selected from Al, Cr, Ta, Ti, Mo, or W,an alloy containing any of these elements as a component, an alloycontaining any of these elements in combination, or the like can beused. The conductive film can be a single layer or a stack formed usingsilicon, a metal material such as copper, neodymium, or scandium, or analloy material containing any of these materials as a main component, inaddition to the above metal. For example, the gate electrode 101 a andthe first wiring 101 b can be formed to have a stacked-layer structurein which an Al film is sandwiched between Ti films.

The gate electrode can also be formed using a light-transmittingconductive film instead of a non-light-transmitting metal film. As alight-transmitting conductive film, a transparent conductive oxide orthe like can be given.

Next, the gate insulating layer 102 and the oxide semiconductor film 103are formed. The gate insulating layer 102 can be formed by a plasma CVDmethod or a sputtering method. The gate insulating layer 102 is formedusing an oxide insulating film such as silicon oxide or aluminum oxide.A silicon oxide film and an aluminum oxide film can be formed by asputtering method. For example, in the case where a silicon oxide filmis formed by a sputtering method, a silicon target or a quartz target isused as a target, and oxygen or a mixed gas of oxygen and argon is usedas a sputtering gas. Since the amount of hydrogen in a silicon oxidefilm or an aluminum oxide film formed by a sputtering method is small,the silicon oxide film or the silicon aluminum film formed by asputtering method is preferably used as the gate insulating layer 102 incontact with the oxide semiconductor film 103. When a large amount ofhydrogen is contained in the gate insulating layer 102, hydrogen isdiffused into the oxide semiconductor film 103, which causes change incharacteristics of the transistor.

Note that the gate insulating layer 102 can be formed to have astructure in which a silicon nitride layer and a silicon oxide layer arestacked from the gate electrode 101 a side. For example, a siliconnitride layer (SiN_(y) (y>0)) is formed as the first gate insulatinglayer 102 a by a sputtering method, and a silicon oxide layer (SiO_(x)(x>0)) is stacked as the second gate insulating layer 102 b over thefirst gate insulating layer 102 a; in such a manner, the gate insulatinglayer 102 having a thickness of 100 nm is formed as illustrated in FIG.2A.

It is preferable that hydrogen, a hydroxyl group, and moisture becontained in the gate insulating layer 102 as little as possible;therefore, the substrate 100 over which the gate electrode 101 a and thefirst wiring 101 b are formed is preferably heated at 200° C. or higherin a preheating chamber of a sputtering apparatus as pretreatment fordeposition, so that impurities attached to the substrate 100 can beremoved.

Then, the oxide semiconductor film 103 is formed over the gateinsulating layer 102. The oxide semiconductor film 103 is formed by asputtering method. The oxide semiconductor film 103 is formed using, forexample, an In—Ga—Zn—O-based non-single-crystal film, anIn—Sn—Zn—O-based oxide semiconductor film, an In—Al—Zn—O-based oxidesemiconductor film, a Sn—Ga—Zn—O-based oxide semiconductor film, anAl—Ga—Zn—O-based oxide semiconductor film, a Sn—Al—Zn—O-based oxidesemiconductor film, an In—Zn—O-based oxide semiconductor film, aSn—Zn—O-based oxide semiconductor film, an Al—Zn—O-based oxidesemiconductor film, an In—O-based oxide semiconductor film, a Sn—O-basedoxide semiconductor film, or a Zn—O-based oxide semiconductor film.Further, the oxide semiconductor film 103 can be formed by a sputteringmethod under a rare gas (typically argon) atmosphere, an oxygenatmosphere, or an atmosphere containing a rare gas (typically argon) andoxygen. When a sputtering method is employed, deposition may beperformed using a target containing silicon oxide at greater than orequal to 2 wt % and less than or equal to 10 wt % so that silicon oxideis contained in the film. Since silicon oxide prevents crystallizationof the oxide semiconductor film, the oxide semiconductor film containingsilicon oxide can be prevented from being crystallized during heattreatment for dehydration or dehydrogenation in a later step.

As a target for forming the oxide semiconductor film 103 by a sputteringmethod, a metal oxide target containing zinc oxide as its main componentcan be used. As another example of a metal oxide target, an oxidesemiconductor target containing In, Ga, and Zn (in a composition ratio,In₂O₃:Ga₂O₃:ZnO=1:1:1 [mol %], In:Ga:Zn=1:1:0.5 [atomic %]) can be used.The fill rate of the oxide semiconductor target is greater than or equalto 90% and less than or equal to 100%, preferably, greater than or equalto 95% and less than or equal to 99.9%. With the use of the oxidesemiconductor target with high fill rate, a dense oxide semiconductorfilm is formed.

In the case where the oxide semiconductor film 103 is formed by asputtering method, first, the substrate is held in a treatment chamberwhich is kept in a reduced-pressure state, and the substrate is heatedat lower than 400° C. Then, a sputtering gas from which hydrogen andmoisture are removed is introduced into the treatment chamber from whichremaining moisture is being removed, and an oxide semiconductor film isdeposited over the substrate with use of a metal oxide as a target. Inorder to remove remaining moisture in the treatment chamber, anentrapment vacuum pump is preferably used. For example, a cryopump, anion pump, or a titanium sublimation pump is preferably used. Theevacuation unit may be a turbo pump provided with a cold trap. In thedeposition chamber which is evacuated with the cryopump, a hydrogenatom, a compound containing a hydrogen atom such as H₂O, a compoundcontaining a carbon atom, and the like are removed, whereby theconcentration of impurities in the oxide semiconductor film formed inthe deposition chamber can be reduced.

There is no particular limitation on the deposition condition of theoxide semiconductor film, and it is set as appropriate. As one exampleof the deposition condition, the distance between the substrate and thetarget is 100 mm, the pressure is 0.6 Pa, the direct-current (DC) powersource is 0.5 kW, and the atmosphere is an oxygen atmosphere (theproportion of the oxygen flow rate is 100%). Note that a pulsedirect-current (DC) power source is preferably used because dust can bereduced and the film thickness can be uniform. The thickness of theoxide semiconductor film 103 is preferably greater than or equal to 5 nmand less than or equal to 50 nm. Needless to say, the thickness of theoxide semiconductor film is not limited to the above range, and is setas appropriate in consideration of characteristics of the transistor.

By formation of the oxide semiconductor film by a sputtering method asdescribed above, an oxide semiconductor film whose quantitative resultof the concentration of hydrogen measured by secondary ion massspectrometry (SIMS) is reduced to 2×10¹⁹ cm⁻³ or less, preferably 5×10¹⁸cm⁻³ or less can be obtained.

Here, the concentration of hydrogen in the oxide semiconductor film isdescribed. FIG. 5 shows results of measuring, by a secondary ion massspectrometry method, the concentration of hydrogen in oxidesemiconductor films formed by a sputtering method.

In FIG. 5, an oxide semiconductor film formed during evacuation with acryopump is shown as an Example. This oxide semiconductor film is formedunder the following condition: a metal oxide target (manufactured byKojundo Chemical Lab. Co., Ltd, the relative density: 80%) having aratio of In:Ga:Zn=1:1:0.5 is used as a target; argon and oxygen are used(Ar/O₂=30/15 sccm) as a sputtering gas; the T/S distance is 60 mm; theDC electric power is 0.5 kW; and the deposition pressure is 0.4 Pa.

In FIG. 5, an oxide semiconductor film formed during evacuation with aturbo molecular pump using a target which has the same composition andis formed by the same manufacturer as the above example is shown as acomparative example. This oxide semiconductor film is formed under thefollowing condition: argon and oxygen (Ar/O₂=10/5 sccm) are used as asputtering gas; the T/S distance is 170 mm; the DC electric power is 0.5kW; and the deposition pressure is 0.4 Pa.

As apparent from FIG. 5, the concentration of hydrogen in the film of asample of the example (i.e., the oxide semiconductor film formed duringevacuation with a cryopump) is reduced to 5×10¹⁹/cm³ or less, whereasthe concentration of hydrogen in the film of a sample of the comparativeexample is 1×10²⁰/cm³ or more. It is apparent from FIG. 5 that theconcentration of hydrogen in the oxide semiconductor film can be morereduced in the case where deposition by a sputtering method is performedwith an entrapment vacuum pump such as a cryopump removing moisture.

FIG. 2B shows the stage at which the oxide semiconductor film 103 isprocessed into an island shape by a second photolithography step to forman oxide semiconductor layer 113, and the contact hole 108 is formed bya third photolithography step. A resist mask for forming thesemiconductor layer 113 may be formed by an inkjet method; in that case,the manufacturing cost can be reduced because a photomask is not used.

The contact hole 108 is provided in the gate insulating layer 102 toexpose the first wiring 101 b.

FIG. 2C shows the stage at which a conductive film to be the sourceelectrode, the drain electrode, and the like of the thin film transistoris formed. As the conductive film, a metal selected from Ti, Mo, W, Al,Cr, Cu, or Ta, an alloy containing any of the above elements as acomponent, an alloy containing any of these elements in combination, orthe like can be used. The conductive film is not limited to a singlelayer containing the above-described element and may be a stack of twoor more layers. For example, a three-layer conductive film in which atitanium film (with a thickness of 100 nm), an aluminum film (with athickness of 200 nm), and a titanium film (with a thickness of 100 nm)are stacked is formed. Instead of a titanium film, a titanium nitridefilm may be used.

Next, a fourth photolithography step is performed. A resist mask isformed, and the conductive film is selectively etched, so that thesource electrode 104 a, the drain electrode 104 b, and the second wiring104 c are formed. In FIG. 2C, the second wiring 104 c is in contact withthe first wiring 101 b through the contact hole 108 which is formed inthe first gate insulating layer 102 a and the second gate insulatinglayer 102 b.

In the fourth photolithography step, only portions of the conductivefilm which are in contact with the oxide semiconductor layer 113 areselectively removed. In the case of using an ammonia peroxide mixture(at a composition weight ratio of hydrogen peroxide:ammonia:water=5:2:2)or the like as an alkaline etchant in order to selectively remove onlyportions of the conductive film which are in contact with the oxidesemiconductor layer 113, the metal conductive film can be selectivelyremoved, so that the oxide semiconductor layer 123 containing anIn—Ga—Zn—O-based oxide semiconductor can remain. A resist mask forforming the source electrode 104 a, the drain electrode 104 b, and thesecond wiring 104 c may be formed by an inkjet method. Formation of theresist mask by an inkjet method needs no photomask; thus, manufacturingcost can be reduced.

FIG. 2D shows the stage at which the oxide insulating film 105 and theprotective insulating film 106 are formed over the oxide semiconductorlayer 123 over which the source electrode 104 a, the drain electrode 104b, and the second wiring 104 c are formed.

In a region where the oxide semiconductor layer 123 overlaps withneither the source electrode 104 a nor the drain electrode 104 b, theoxide semiconductor layer 123 and the oxide insulating film 105 are incontact with each other. A region of the oxide semiconductor layer 123which is between the source electrode 104 a and the drain electrode 104b, overlaps with the gate electrode, and is provided between the oxideinsulating film 105 and the gate insulating layer 102 and in contactwith the oxide insulating film 105 and the gate insulating layer 102serves as a channel formation region.

The oxide insulating film 105 is preferably formed by a sputteringmethod. This is because hydrogen is not to be contained in the oxideinsulating film 105 like the above gate insulating layer. Therefore, theoxide insulating film 105 is also preferably formed by the sputteringmethod in the state where remaining hydrogen, a remaining hydroxylgroup, or remaining moisture in the treatment chamber is removed. As amaterial for forming the oxide insulating film 105, silicon oxide oraluminum oxide can be used.

There is no particular limitation on the deposition condition of theoxide insulating film, and it is set as appropriate. For example, asilicon oxide film is formed by a pulse DC sputtering method under thefollowing condition: a boron-doped silicon target (having a resistanceof 0.01 Ωcm) is used; the distance between the substrate and the target(the T-S distance) is 89 mm; the pressure is 0.4 Pa; the direct-current(DC) power source is 6 kW; and the atmosphere is an oxygen atmosphere(the proportion of the oxygen flow rate is 100%). By formation of thesilicon oxide film by a sputtering method, a defect (due to a structuraldefect, i.e., a dangling bond) can be included in the silicon oxidefilm. This is because a sputtering method is a physical depositionmethod in which atoms or particles sputtered from a target are made tobe attached to a substrate, and the atoms or particles attached to thesubstrate are rapidly cooled, so that a thin film including a structuraldefect is easily formed. Note that instead of a silicon target, quartz(preferably, synthetic quartz) can be used as the target for forming thesilicon oxide film.

Next, the protective insulating film 106 is formed over the oxideinsulating film 105. As the protective insulating film 106, a siliconnitride film, a silicon nitride oxide film, an aluminum nitride film, orthe like can be used. Here, as the protective insulating film 106, asilicon nitride film is formed. The silicon nitride film can be formedby a sputtering method.

When the protective insulating film 106 is formed, the substrate 100 isheated at 200° C. to 400° C.; thus, hydrogen or moisture included in theoxide semiconductor film can be diffused into the oxide insulating film(the silicon oxide film including a defect). Since the oxide insulatingfilm 105 contains many defects (dangling bonds), an impurity such ashydrogen, a hydroxyl group, or moisture contained in the oxidesemiconductor layer 123 are diffused into the oxide insulating film 105through the interface between the oxide semiconductor layer 123 and theoxide insulating film 105. Specifically, a hydrogen atom, a compoundcontaining a hydrogen atom such as H₂O, or a compound containing acarbon atom in the oxide semiconductor layer 123 is easily diffused intothe oxide insulating film 105.

When the oxide semiconductor film is formed in the above manner,remaining moisture in a reaction atmosphere is removed; thus, theconcentration of hydrogen and that of hydride in the oxide semiconductorfilm can be reduced. Therefore, characteristics of the transistor can bestabilized.

When the oxide semiconductor film serving as the channel formationregion is formed over the gate insulating film, remaining moisture in areaction atmosphere is removed; thus, the concentration of hydrogen andthat of hydride in the oxide semiconductor film can be reduced. Byprovision of the silicon oxide film including a defect in contact withthe oxide semiconductor film, hydrogen and moisture in the oxidesemiconductor film can be diffused into the silicon oxide film, so thatthe concentration of hydrogen and that of hydride in the oxidesemiconductor film can be reduced.

In addition, the silicon nitride film is formed over the silicon oxidefilm including a defect in the state where the substrate is heated;thus, hydrogen and moisture can be diffused from the oxide semiconductorfilm into the silicon oxide film and a barrier film for prevention ofentry of moisture from an outer atmosphere can be provided.

The above steps can be used for manufacture of a liquid crystal displaypanel, an electroluminescence display panel, and a backplane (asubstrate over which a thin film transistor is formed) of a displaydevice using electronic ink. The above steps are performed at 400° C. orlower; therefore, the above steps can be applied to a manufacturingprocess in which a glass substrate having a thickness of 1 mm or lessand having a side that is longer than 1 m is used. All the above stepscan be performed at 400° C. or lower; thus, a large amount of energy isnot needed for manufacturing a display panel.

FIG. 3 illustrates an example of a deposition apparatus that can be usedfor manufacturing an oxide semiconductor film and a semiconductor devicein which an oxide semiconductor film is used.

The deposition apparatus illustrated in FIG. 3 is provided with a loadchamber 207 and an unload chamber 208. In each of the load chamber 207and the unload chamber 208, a cassette 210 which stores a substratebefore treatment or a substrate after treatment is provided. A firsttransfer chamber 201 is provided between the load chamber 207 and theunload chamber 208, and is provided with a transfer unit 209 whichtransfers a substrate.

The deposition apparatus is provided with a second transfer chamber 202.The second transfer chamber 202 is provided with the transfer unit 209.Four treatment chambers (a first treatment chamber 203, a secondtreatment chamber 204, a third treatment chamber 205, and a fourthtreatment chamber 206) are connected to the second transfer chamber 202through gate valves, and are arranged around the second transfer chamber202. Note that one side of the first treatment chamber 203 is connectedto the first transfer chamber 201 through a gate valve, and the otherside of the first treatment chamber 203 is connected to the secondtransfer chamber 202 through a gate valve.

The second transfer chamber 202, the first treatment chamber 203, thesecond treatment chamber 204, the third treatment chamber 205, and thefourth treatment chamber 206 are each provided with an evacuation unit212. Although the evacuation unit may be selected depending on the useapplication of each treatment chamber, an entrapment evacuation unitsuch as a cryopump is particularly preferable. Alternatively, as theevacuation unit, a turbo pump provided with a cold trap may be used.Such an evacuation unit has an effect of attaching remaining moisture ina treatment chamber to a cooled metal surface; therefore, it iseffective to use such an evacuation unit for improvement in purity of anoxide semiconductor film.

In the case where the oxide semiconductor film is formed, an evacuationunit such as a cryopump is preferably used in order to preventincorporation of remaining moisture as an impurity in the treatmentchambers (needless to say, including the treatment chamber for formingthe oxide semiconductor film) in steps before and after formation offilms in contact with the oxide semiconductor film and steps before andafter formation of the oxide semiconductor film.

A substrate-heating unit 211 is provided in the first treatment chamber203. The first treatment chamber 203 serves as a delivery chamber fortransferring a substrate from the first transfer chamber 201 in anatmospheric-pressure state to the second transfer chamber 202 in areduced-pressure state. By provision of the delivery chamber, the secondtransfer chamber 202 can be prevented from being contaminated by air.

The second treatment chamber 204, the third treatment chamber 205, andthe fourth treatment chamber 206 are provided with a structure forforming a silicon nitride film by a sputtering method, a structure forforming a silicon oxide film by a sputtering method, and a structure forforming an oxide semiconductor film by a sputtering method,respectively. That is, a target and a substrate-heating unit areprovided in each of the treatment chambers, and the treatment chambersare each provided with a gas supply unit for introducing a sputteringgas and a glow discharge generation unit.

Next, an example of operation of the deposition apparatus is described.Here, a method for successively forming the gate insulating film and theoxide semiconductor film over the substrate over which the gateelectrode 101 a and the first wiring 101 b are formed as illustrated inFIG. 2A is described.

The transfer unit 209 transfers the substrate 100 over which the gateelectrode 101 a and the first wiring 101 b are formed to the firsttreatment chamber 203 from the cassette 210. Next, the substrate 100 ispreheated in the first treatment chamber 203 with the gate valve closed,whereby impurities attached to the substrate are eliminated andevacuated. Examples of the impurities are a hydrogen atom, a compoundcontaining a hydrogen atom such as H₂O, a compound containing a carbonatom, and the like.

The substrate 100 is transferred to the second treatment chamber 204,and a silicon nitride film is formed. Then, the substrate 100 istransferred to the third treatment chamber 205, and a silicon oxide filmis formed. In such a manner, the gate insulating layer 102 is formed.The second treatment chamber 204 and the third treatment chamber 205 arepreferably evacuated by a cryopump or the like so that the concentrationof impurities in the treatment chambers can be reduced. The siliconnitride film and the silicon oxide film stacked in the treatmentchambers in which the concentration of impurities is reduced are used asthe gate insulating layer 102 in which hydrogen, a hydroxyl group,moisture, or the like is reduced.

The substrate 100 is transferred to the third treatment chamber 205. Atarget for an oxide semiconductor is provided in the third treatmentchamber 205, and the third treatment chamber 205 is provided with acryopump as an evacuation unit. In the third treatment chamber 205, anoxide semiconductor layer is formed. In the third treatment chamber 205,remaining moisture is removed by the cryopump, so that the concentrationof hydrogen in the oxide semiconductor film 103 can be reduced. Theoxide semiconductor film 103 is formed in the state where the substrateis heated. Deposition by a sputtering method is performed in the statewhere remaining moisture in the treatment chamber is removed by thecryopump, whereby the substrate temperature at the time of forming theoxide semiconductor film 103 can be 400° C. or lower.

In the above-described manner, the gate insulating layer 102 and theoxide semiconductor film 103 can be successively formed by thedeposition apparatus. The structure in which three or more treatmentchambers are connected through a transfer chamber is employed in FIG. 3;however, another structure may be employed. For example, a so-calledin-line structure in which the entrance and the exit for the substrateare provided and the treatment chambers are connected to each other maybe employed.

FIG. 4 shows an example of a deposition apparatus for forming the oxideinsulating film 105 and the protective insulating film 106 over theoxide semiconductor layer 123 as illustrated in FIG. 2D.

This deposition apparatus is provided with a load chamber 307 and anunload chamber 308. The load chamber 307 and the unload chamber 308 areeach provided with a cassette 310 which stores a substrate beforetreatment or a substrate after treatment.

In addition, this deposition apparatus is provided with a transferchamber 301. The transfer chamber 301 is provided with a transfer unit309. Five treatment chambers (a first treatment chamber 302, a secondtreatment chamber 303, a third treatment chamber 304, a fourth treatmentchamber 305, and a fifth treatment chamber 306) are connected to thetransfer chamber 301 through gate valves, and are arranged around thetransfer chamber 301.

The load chamber 307, the unload chamber 308, the transfer chamber 301,the first treatment chamber 302, the second treatment chamber 303, thethird treatment chamber 304, the fourth treatment chamber 305, and thefifth treatment chamber 306 are each provided with an evacuation unit313, so that the chambers can be in a reduced-pressure state. Althoughthe evacuation unit may be selected in accordance with the useapplication of each treatment chamber, an evacuation unit such as acryopump is particularly preferable. Alternatively, as the evacuationunit, a turbo pump provided with a cold trap may be used.

The load chamber 307 and the unload chamber 308 each serve as a deliverychamber for transferring a substrate to the transfer chamber 301. Byprovision of the delivery chamber, the transfer chamber 301 can beprevented from being contaminated by air.

The first treatment chamber 302 and the fourth treatment chamber 305 areeach provided with a substrate-heating unit 311. The second treatmentchamber 303 and the third treatment chamber 304 are provided with astructure for forming a silicon oxide film by a sputtering method and astructure for forming a silicon nitride film by a sputtering method,respectively. That is, a target and a substrate-heating unit areprovided in each of the treatment chambers, and the treatment chambersare each provided with a gas supply unit for introducing a sputteringgas and a glow discharge generation unit. In addition, a cooling unit312 is provided in the fifth treatment chamber 306.

An example of operation of the deposition apparatus is described. Amethod for forming the oxide insulating film 105 and the protectiveinsulating film 106 over the oxide semiconductor layer 123 asillustrated in FIG. 2D is described.

First, the load chamber 307 is evacuated so that the load chamber 307 ismade to have substantially the same pressure as the transfer chamber301, and then, the substrate 100 is transferred from the load chamber307 to the first treatment chamber 302 through the transfer chamber 301with the gate valve opened.

It is preferable that the substrate 100 be preheated by thesubstrate-heating unit 311 in the first treatment chamber 302 so thatimpurities attached to the substrate can be eliminated and evacuated.Examples of the impurities are a hydrogen atom, a compound containing ahydrogen atom such as H₂O, a compound containing a carbon atom, and thelike. Note that the temperature of the preheating is higher than orequal to 100° C. and lower than or equal to 400° C., preferably higherthan or equal to 150° C. and lower than or equal to 300° C. As anevacuation unit provided for the first treatment chamber 302, a cryopumpis preferably used. Since impurities attached to the substrate 100 areeliminated by the preheating and are diffused into the first treatmentchamber 302, the impurities should be evacuated from the first treatmentchamber 302 with use of a cryopump. Note that this preheating treatmentcan be omitted.

The substrate 100 is transferred to the second treatment chamber 303,and the oxide insulating film 105 is formed. For example, a siliconoxide film is formed as the oxide insulating film 105. The secondtreatment chamber 303 is evacuated by a cryopump or the like, so thatthe concentration of impurities in the treatment chamber is reduced. Theconcentration of impurities in the oxide insulating film formed in thetreatment chamber with reduced impurities is suppressed. Specifically,the concentration of hydrogen contained in the oxide insulating film canbe reduced. Although the oxide insulating film 105 may be formed in thestate where the substrate 100 is heated, the oxide insulating film 105is preferably formed at room temperature to about 200° C. so that theoxide insulating film 105 includes a defect.

In the case where a silicon oxide film is formed as the oxide insulatingfilm 105 by a sputtering method, a quartz target or a silicon target canbe used as a target. The silicon oxide film formed by a sputteringmethod under an atmosphere including oxygen and a rare gas with use of aquartz target or a silicon target can include a dangling bond of asilicon atom or an oxygen atom.

By provision of the oxide insulating film 105 including a large numberof dangling bonds in contact with the oxide semiconductor layer 123,impurities such as hydrogen, a hydroxyl group, and moisture in the oxidesemiconductor layer 123 are easily diffused into the oxide insulatingfilm 105 through the interface between the oxide semiconductor layer 123and the oxide insulating film 105. Specifically, a hydrogen atom or acompound containing a hydrogen atom such as H₂O in the oxidesemiconductor layer 123 is easily diffused into the oxide insulatingfilm 105. As a result, the concentration of impurities in the oxidesemiconductor layer 123 is reduced.

Next, the substrate 100 is transferred to the third treatment chamber304, and the protective insulating film 106 is formed over the oxideinsulating film 105. As the protective insulating film 106, a filmhaving a function of preventing diffusion of impurity elements is used;for example, a silicon nitride film, a silicon nitride oxide film, orthe like can be used. The third treatment chamber 304 is preferablyevacuated by a cryopump or the like so that the concentration ofimpurities in the treatment chamber can be reduced.

The protective insulating film 106 prevents diffusion and entry ofimpurities from an outer atmosphere of the thin film transistor 110.Examples of the impurities are hydrogen, a compound containing ahydrogen atom such as H₂O, a compound containing a carbon atom, and thelike.

In the case where a silicon nitride film is formed as the protectiveinsulating film 106 by a sputtering method, for example, the protectiveinsulating film 106 is formed in the following manner: a silicon targetis used; a mixed gas of nitrogen and argon is introduced into the thirdtreatment chamber 304; and reactive sputtering is performed. Thesubstrate temperature is set to higher than or equal to 200° C. andlower than or equal to 400° C., for example, higher than or equal to200° C. and lower than or equal to 350° C. Through the deposition athigh temperature, impurities including a hydrogen atom such as hydrogen,a hydroxyl group, and moisture can be diffused into the oxide insulatingfilm 105 and the concentration of impurities in the oxide semiconductorlayer 123 can be reduced. In particular, the substrate temperature ispreferably higher than or equal to 200° C. and lower than or equal to350° C. so that diffusion of hydrogen atoms can be promoted.

Note that the heat treatment may be performed after the protectiveinsulating film 106 is formed in order to diffuse impurities including ahydrogen atom such as hydrogen, a hydroxyl group, and moisture into theoxide insulating film 105 and to reduce the concentration of impuritiesin the oxide semiconductor layer 123.

For example, as illustrated in FIG. 4, the substrate 100 is transferredto the fourth treatment chamber 305, and heat treatment after depositionis performed. The substrate temperature of the heat treatment afterdeposition is higher than or equal to 200° C. and lower than or equal to400° C. Through the heat treatment, impurities included in the oxidesemiconductor layer can be easily diffused into the oxide insulatingfilm 105 through the interface between the oxide semiconductor layer 123and the oxide insulating film 105. Specifically, a hydrogen atom or acompound containing a hydrogen atom such as H₂O in the oxidesemiconductor layer 123 is easily diffused into the oxide insulatingfilm. As a result, the concentration of impurities in the oxidesemiconductor layer 123 is reduced.

After the heat treatment, the substrate 100 is transferred to the fifthtreatment chamber 306. The substrate 100 is cooled to such lowtemperature that reincorporation of impurities such as water issuppressed from the substrate temperature T of the heat treatment afterthe deposition. Specifically, the substrate 100 is slowly cooled to thetemperature lower than the substrate temperature T by 100° C. or more.Cooling may be performed with helium, neon, argon, or the likeintroduced into the fifth treatment chamber 306. Note that it ispreferable that water, hydrogen, or the like be not included in nitrogenor a rare gas such as helium, neon, or argon which is used for thecooling. The purity of nitrogen or a rare gas such as helium, neon, orargon is preferably 6N (99.9999%) or more, more preferably 7N(99.99999%) or more (that is, the concentration of impurities is 1 ppmor less, preferably 0.1 ppm or less).

With use of a deposition apparatus to which an evacuation unit such as acryopump is applied, impurities in a treatment chamber can be reduced.Impurities attached to the inner wall of the treatment chamber areeliminated, and incorporation of impurities into a substrate duringdeposition and a film can be reduced. In addition, impurities which areeliminated from an atmosphere during preheating are evacuated, wherebythe impurities can be prevented from being attached to the substrateagain.

The oxide insulating film 105 formed in the above manner includes alarge number of dangling bonds. By provision of the oxide insulatingfilm 105 in contact with the oxide semiconductor layer 123, impuritiessuch as a hydrogen atom and a compound containing a hydrogen atom suchas H₂O in the oxide semiconductor layer 123 can be diffused into ormoved to the oxide insulating film 105 from the oxide semiconductorlayer 123. As a result, the concentration of impurities in the oxidesemiconductor layer 123 can be reduced.

For example, in a thin film transistor in which an oxide semiconductorlayer serving as a channel formation region is in contact with an oxideinsulating film formed using a deposition apparatus described in thisembodiment, the carrier concentration of the channel formation region isreduced in the state where voltage is not applied to a gate electrode,i.e., in the off state; therefore, the thin film transistor has low offcurrent and has favorable characteristics.

A structure in which three or more treatment chambers are connectedthrough a transfer chamber is employed in FIG. 4; however, a structureof an apparatus for reducing hydrogen and moisture in an oxidesemiconductor film is not limited thereto. For example, a so-calledin-line structure in which the entrance and the exit for the substrateare provided and the treatment chambers are connected to each other maybe employed.

The above steps using the deposition apparatus can be used formanufacture of a liquid crystal display panel, an electroluminescencedisplay panel, and a backplane (a substrate over which a thin filmtransistor is formed) of a display device using electronic ink. Theabove steps using the deposition apparatus are performed at 400° C. orlower; therefore, the above steps using the deposition apparatus can beapplied to a manufacturing process in which a glass substrate having athickness of 1 mm or less and having a side that is longer than 1 m isused. All the above steps can be performed at 400° C. or lower; thus, alarge amount of energy is not needed for manufacturing a display panel.

This application is based on Japanese Patent Application serial no.2009-219210 filed with Japan Patent Office on Sep. 24, 2009, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A method for manufacturing a semiconductordevice, comprising a step of forming an oxide semiconductor film over asubstrate by a sputtering method using an oxide semiconductor target ina chamber, wherein a filling rate of the oxide semiconductor target isgreater than or equal to 90%, wherein the chamber is evacuated with useof an entrapment vacuum pump, and wherein a sputtering gas from whichmoisture is removed is introduced into the chamber.
 2. The method formanufacturing a semiconductor device according to claim 1, wherein theentrapment vacuum pump is a cryopump.
 3. The method for manufacturing asemiconductor device according to claim 1, wherein the entrapment vacuumpump is a turbo molecular pump with a cold trap.
 4. The method formanufacturing a semiconductor device according to claim 1, wherein aconcentration of hydrogen in the oxide semiconductor film is lower thanor equal to 5×10¹⁹/cm³.
 5. The method for manufacturing a semiconductordevice according to claim 1, wherein the oxide semiconductor filmcomprises indium, gallium, and zinc.
 6. The method for manufacturing asemiconductor device according to claim 1, further comprising a step offorming an insulating film over the oxide semiconductor film while thesubstrate is heated at 200° C. to 400° C.
 7. The method formanufacturing a semiconductor device according to claim 6, wherein theinsulating film comprises a material selected from the group consistingof silicon nitride, silicon nitride oxide, and aluminum nitride.
 8. Amethod for manufacturing a semiconductor device, comprising the stepsof: introducing a substrate into a chamber, the chamber being evacuatedwith use of an entrapment vacuum pump; supplying a gas from whichmoisture is removed into the chamber; and forming an oxide semiconductorfilm over the substrate using the gas and an oxide semiconductor target,a filling rate of the oxide semiconductor target being greater than orequal to 90%.
 9. The method for manufacturing a semiconductor deviceaccording to claim 8, wherein the entrapment vacuum pump is a cryopump.10. The method for manufacturing a semiconductor device according toclaim 8, wherein the entrapment vacuum pump is a turbo molecular pumpwith a cold trap.
 11. The method for manufacturing a semiconductordevice according to claim 8, wherein a concentration of hydrogen in theoxide semiconductor film is lower than or equal to 5×10¹⁹/cm³.
 12. Themethod for manufacturing a semiconductor device according to claim 8,wherein the oxide semiconductor film comprises indium, gallium, andzinc.
 13. The method for manufacturing a semiconductor device accordingto claim 8, further comprising a step of forming an insulating film overthe oxide semiconductor film while the substrate is heated at 200° C. to400° C.
 14. The method for manufacturing a semiconductor deviceaccording to claim 13, wherein the insulating film comprises a materialselected from the group consisting of silicon nitride, silicon nitrideoxide, and aluminum nitride.